For accurate statistical analysis of the 2dFGRS it is essential to
fully understand the criteria that define the parent photometric
catalogue and also the completeness of the redshift catalogue, as
discussed in
Colless et al.
(2001) and
Norberg et al.
(2002). For this purpose we have defined maps or masks
characterizing this information as a function of position on the sky:

The magnitude limit mask gives the extinction-corrected magnitude
limit of the survey at each position.

The redshift completeness mask gives the fraction of measured
redshifts at each position.

The mu-mask gives the dependence of the redshift completeness on
apparent magnitude.

We now describe in more detail how these masks are defined and
briefly outline some of their uses.

1. Magnitude limit mask

Although the 2dFGRS sample was originally selected to have a uniform
extinction-corrected magnitude limit of b=19.45, in fact the survey
magnitude limit varies slightly with position on the sky. There are two
reasons for this. First, the photometric calibrations now available are
much more extensive than when the parent 2dFGRS catalogue was originally
defined. This has enabled us to recalibrate the whole 2dFGRS parent
catalogue and results in improved zero-point offsets and linearity
corrections for each of the UKST photographic plates. Second, the
extinction corrections have been changed to use the final published
version of the Schlegel et al. (1998) extinction maps; the original
extinction corrections came from a preliminary version of those maps.

The magnitude limit mask is therefore defined by the change in the
photometric calibration of each UKST photographic plate and the change
in the dust extinction correction at each position on the sky. The
magnitude limit masks for the NGP and SGP strips using the photometric
calibration of the 2dFGRS Final Data Release are shown below in a
zenithal equal area projection. Note that the masks also account for the
holes in the source catalogue around bright stars and plate flaws.

In the SGP, which is a subset of the APM galaxy survey (Maddox et al.
1990a,b), the rms change in plate zero-point is only 0.03 mag.
However, in the NGP region the original calibration was less accurate
and the change in zero-points have an rms of 0.08 mag. The change
in the dust corrections are also less in the SGP, as the extinction is
generally lower in this region. In the SGP the rms magnitude change due
to improved dust corrections is 0.01 mag while in the NGP it is
0.02 mag.

In the SGP the mean limiting magnitude is b=19.40 with an rms
about this value of 0.08 mag; in the NGP the mean limiting
magnitude is b=19.29 with an rms of 0.12 mag.

For accurate statistical analysis of the 2dF survey, the magnitude
limits defined by this mask should be used. It is always possible to
analyse the data with a fixed magnitude limit if one is prepared to omit
both the areas of the survey that have magnitude limits brighter than
the chosen limit and also all the galaxies in the remaining areas with
magnitudes fainter than the chosen limit.

2. Simple redshift completeness mask

The best way to define a redshift completeness mask is to make use of
the geometry defined by the complete set of 2 degree fields that
were used to tile the survey region for spectroscopic observations. Each
region of the sky inside the survey boundary is covered by at least one
2 degree field, but more often by several overlapping fields. We
define a sector as the region delimited by a unique set of overlapping
2 degree fields. This is the most natural way of partitioning the
sky, as it takes account of the geometry imposed by the pattern of
2 degree fields and the way in which the galaxies were targeted for
spectroscopic observation. Within each sector, theta, we define the
redshift completeness, R(theta), as the ratio of the number of galaxies
for which redshifts have been obtained, N_z(theta), to the total number
of objects contained in the parent catalogue, N_p(theta):

R(theta) = N_z(theta)/N_p(theta) .

The redshift completeness of a given sector, R(theta), should be
clearly distinguished from the redshift completeness of a given field,
C_F, since multiple overlapping fields can contribute to a single
sector.

The redshift completeness masks for the 2dFGRS Final Data Release
(where redshift completeness is defined as above) are shown below. The
masks are plotted in a zenithal equal area projection.

These simple redshift completeness masks can be used to locate
regions in which the redshift completeness is high. They can also be
used as a first step in either applying weights to statistically correct
for incompleteness or to construct random unclustered catalogues that
have the same angular pattern of incompleteness as the redshift sample
(for use in estimating correlation functions).

For this latter purpose, one should also take account of how the
redshift completeness depends on position within a sector as a result of
constraints on fibre positioning and other considerations. This is best
done by using the parent catalogue to derive weights for each galaxy
with a measured redshift (see
Norberg et al.
2002).

3. Magnitude-dependent completeness corrections (mu-mask)

For many applications one also needs to take account of how the
redshift completeness depends on apparent magnitude, as discussed in
detail in
Colless et al.
(2001) and
Norberg et al.
(2002). This requires knowing the mu parameter that
characterises the fall-off in completeness with apparent magnitude for
each sector.

Correction: In the definition of the magnitude-dependent
completeness mask in section 8.3 of Colless et al. (2001), there is an
error in the value given for the parameter alpha; the paper gives
alpha=0.5, but the correct value is alpha=0.5ln(10).

The mu-masks for the 2dFGRS Final Data Release are shown below. The
masks are plotted in a zenithal equal area projection.